Ultrafast Dynamics in Aromatic Cation Based Ionic Liquids: A Femtosecond Raman-Induced Kerr Effect Spectroscopic Study

Abstract

Abstract We studied the ultrafast dynamics of 40 aromatic cation based ionic liquids (ILs) by means of femtosecond Raman-induced Kerr effect spectroscopy. The low-frequency Kerr spectra (ca. 0.3–700 cm−1) of the ILs were obtained from the Kerr transients by Fourier-transform deconvolution analysis. The low-frequency Kerr spectra in the frequency range less than 200 cm−1 coming mainly from the intermolecular vibrations for the ILs were discussed in terms of (i) anion dependence, (ii) imidazolium cations vs. pyridinium cations, (iii) alkyl group dependence, and (iv) effect of methylation in aromatic cations. Several liquid properties, such as density, viscosity, electrical conductivity, and surface tension, of the present sample ILs at 293 K were also estimated in this study. We clarified that the aromatic cation based ILs show a different relation of the first moment of the low-frequency spectral band to the bulk liquid parameter, which is the square root of surface tension divided by liquid density, from aprotic molecular liquids. The slope of the first moment to the bulk parameter for the aromatic cation based ILs is gentler than that for aprotic molecular liquids.